Photographic materials having tabular AgX grains have improved color sensitivity, sharpness, light-scattering characteristics, covering power, rate of development and graininess than those having non-tabular AgX grains. Accordingly, tabular AgX grains having twin planes parallel to each other and having, as the main planes, {111} planes have become used generally. For the details of such grains, the disclosures in JP-A-58-113926, JP-A-58-113927, JP-A-58-113928 and JP-A-2-28638 (corresponding to U.S. Pat. Nos. 4,439,520, 4,434,226 4,434,226 and 4,945,037, respectively), and JP-A-2-838 and JP-A-2-298935 are referred to. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, where AgX grains adsorb large amounts of sensitizing dyes, in general, those having {100} planes have better color-sensitizing characteristics. Therefore, it is desired to develop tabular AgX grains having, as the main planes, {100} planes. Such {100} tabular grains having right-angled, parallelogramic main planes are described in, for example, JP-A-51-88017 and JP-B-64-8323 (corresponding to U.S. Pat. Nos. 4,063,951 and 4,386,156, respectively), and EP-A-0534395. (The term "JP-B" as used herein means an "examined Japanese patent publication".) However, none of these grains have a discontinuous halogen composition gap plane in their central part. Instead, these grains have a uniform halogen composition or a gradually varying halogen compositions throughout. It is difficult to get good controllability of tabular grain production so that the dispersion of the intended grains is significant in preparing them and, additionally, only AgX emulsions having broad grain size distributions are prepared. These conventional AgX grains are not satisfactory with respect to their sensitivity and graininess and also their ability of forming high-quality photographic images.